1. Field of the Invention
The present invention relates to a constant-speed control device for a vehicle. More particularly, it relates to an improvement of such a device by which a sudden forward running or braked movement (hereinafter surging) of a vehicle when running on a downward slope is prevented.
2. Description of the Related Art
A known constant-speed control device for a vehicle is constructed in such a manner that a target speed is set by a switching operation and a throttle valve is controlled so that the vehicle speed reaches the target speed. In this connection, there is also known a fuel cut device which prevents a fuel supply to the engine, for an improvement of the fuel consumption, when the vehicle is driven at relatively high engine revolutions and the throttle valve is closed to decelerate the vehicle.
In an internal combustion engine provided with both the constant-speed control device and the fuel cut device, a case can occur in which, while the vehicle is running on a downward slope under a constant-speed control, the throttle valve closes the intake passage. In such a case, if the fuel is cut by the fuel cut device, the vehicle speed is suddenly decreased, and the vehicle speed becomes slower than the target speed, and as a result, the throttle valve is opened by the constant-speed control device, so that the fuel supply is resumed. Then, since the vehicle speed is suddenly increased, the throttle valve is moved to close the intake passage again, so that the fuel supply is cut, and thus the vehicle speed is again suddenly decreased. This action is repeated, and thus the engine torque is largely changed due to the opening and closing of the throttle valve. This causes sudden increases and decreases in the vehicle speed, i.e., surging of the vehicle occurs.